[R-sig-Geo] spatial projections

[Arien Lam]

Hello,


On 15-09-10 22:47, James Stegen wrote:
> Hello,
>
> I have lat-long coordinates for a bunch of localities spread across
> multiple hemispheres, which also extend into high latitudes.
That's two hemispheres max, right?

> What I would like to do is re-project the lat long coordinates into a
> projection that gives me spatial positions in km from some arbitrary point.
> The key is that I would like the distances among communities to be
> maintained.Specifically, if I calculate a distance matrix using lat
> longs in the
> great circle equation, I would like a distance matrix calculated from
> the reprojected positions to give the same distances.

That sets the requirements of the projection to equidistant, conformal 
and equal-area. If you read up on projections, even if it is on 
wikipedia, you will soon see that it can't be done. To be precise, it is 
possible in cases with less than three communities.

> I haven't found a projection that can do this. Using the orthographic
> projection, for example, leads to larger distances among sites that
> occur at high latitudes,
> relative to the distances provided by the great circle equation. I also
> need to know actual spatial positions in both axes, not just overall
> distance, relative to
> an arbitrary point on earth.

Hth, Arien

>
> Many Thanks,
> James Stegen
>


-- 
Arien Lam